1. What is Weightlessness?
Weightlessness is the condition in which a person or object experiences no apparent weight. This does not mean gravity is absent—in fact, gravity acts everywhere. Instead, weightlessness happens when there is no normal reaction force acting on the body.
In simple words, you feel weightless when you and your surroundings are falling freely together.
1.1. Everyday Example
If you jump inside a moving lift that suddenly falls (free fall), you would feel your feet lose contact with the floor. This is a brief experience of weightlessness.
1.1.1. Key Idea
You feel weight only when something pushes back on you (like the floor). Without this push, you feel weightless.
2. Why Do Astronauts Feel Weightless?
A common misconception is that astronauts feel weightless because there is no gravity in space. But this is not true.
Satellites and spacecraft orbit the Earth because gravity is pulling them toward the planet. Yet astronauts inside feel weightless because they are in continuous free fall.
2.1. Orbiting is Continuous Free Fall
A satellite is constantly falling toward Earth due to gravity. But it also moves forward fast enough that it keeps missing the Earth. This combination creates an orbit.
2.2. Inside the Spacecraft
Astronauts inside the spacecraft fall at the same rate as the spacecraft itself. Since both fall together, astronauts do not press against the floor or walls. With no normal reaction force, they feel weightless.
3. Weight vs Apparent Weight
Your actual weight is the gravitational force acting on your body:
\( W = mg \)
But what you feel as weight is the apparent weight, which depends on the support force from the surface beneath you.
3.1. Apparent Weight Becomes Zero
In free fall, the support force becomes zero, so your apparent weight becomes zero—even though gravity still acts.
3.2. Normal Reaction Force
Weightlessness occurs when the normal reaction force \( N = 0 \). This is why astronauts float.
4. Situations Where Weightlessness is Experienced
Weightlessness is not limited to space. It can occur in several situations.
4.1. Inside an Orbiting Satellite
The most common example—astronauts in space experience continuous weightlessness.
4.2. Free Fall on Earth
If a lift cable breaks, the lift and everything inside experience temporary weightlessness until air resistance becomes significant.
4.3. Parabolic Flight
Aircraft used for astronaut training follow a curved 'parabolic' path. During part of the flight, passengers inside experience weightlessness for about 20–30 seconds.
5. Why Weightlessness Does NOT Mean Zero Gravity
Even at the altitude of the International Space Station (around 400 km above Earth), gravity is only slightly weaker than on the surface—about 90% of its surface value. So astronauts still experience strong gravitational pull.
5.1. The Real Reason for Floating
Astronauts float because they are falling around Earth, not because gravity is absent.
6. Effects of Long-Term Weightlessness on the Human Body
Extended periods of weightlessness can affect the human body in many ways:
- Muscle weakening
- Reduction in bone density
- Fluid shift towards the head
- Difficulty in sleeping
- Changes in heart function
6.1. Countermeasures
Astronauts exercise daily and use specially designed equipment to reduce the negative effects of long-term exposure to weightlessness.
7. Why Understanding Weightlessness is Important
Weightlessness is essential to modern space exploration. It helps us understand:
- How astronauts live and work in space
- The design of spacecraft and satellites
- The behaviour of fluids and objects in microgravity
- The challenges of long-term space travel
This completes the chapter on Gravitation. Each of these topics helps us understand how bodies attract each other, how planets move, and how gravity shapes our universe.